1==========================
2Kprobe-based Event Tracing
3==========================
4
5:Author: Masami Hiramatsu
6
7Overview
8--------
9These events are similar to tracepoint based events. Instead of Tracepoint,
10this is based on kprobes (kprobe and kretprobe). So it can probe wherever
11kprobes can probe (this means, all functions except those with
12__kprobes/nokprobe_inline annotation and those marked NOKPROBE_SYMBOL).
13Unlike the Tracepoint based event, this can be added and removed
14dynamically, on the fly.
15
16To enable this feature, build your kernel with CONFIG_KPROBE_EVENTS=y.
17
18Similar to the events tracer, this doesn't need to be activated via
19current_tracer. Instead of that, add probe points via
20/sys/kernel/debug/tracing/kprobe_events, and enable it via
21/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enable.
22
23You can also use /sys/kernel/debug/tracing/dynamic_events instead of
24kprobe_events. That interface will provide unified access to other
25dynamic events too.
26
27Synopsis of kprobe_events
28-------------------------
29::
30
31  p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS]	: Set a probe
32  r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]	: Set a return probe
33  -:[GRP/]EVENT						: Clear a probe
34
35 GRP		: Group name. If omitted, use "kprobes" for it.
36 EVENT		: Event name. If omitted, the event name is generated
37		  based on SYM+offs or MEMADDR.
38 MOD		: Module name which has given SYM.
39 SYM[+offs]	: Symbol+offset where the probe is inserted.
40 MEMADDR	: Address where the probe is inserted.
41 MAXACTIVE	: Maximum number of instances of the specified function that
42		  can be probed simultaneously, or 0 for the default value
43		  as defined in Documentation/staging/kprobes.rst section 1.3.1.
44
45 FETCHARGS	: Arguments. Each probe can have up to 128 args.
46  %REG		: Fetch register REG
47  @ADDR		: Fetch memory at ADDR (ADDR should be in kernel)
48  @SYM[+|-offs]	: Fetch memory at SYM +|- offs (SYM should be a data symbol)
49  $stackN	: Fetch Nth entry of stack (N >= 0)
50  $stack	: Fetch stack address.
51  $argN		: Fetch the Nth function argument. (N >= 1) (\*1)
52  $retval	: Fetch return value.(\*2)
53  $comm		: Fetch current task comm.
54  +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
55  \IMM		: Store an immediate value to the argument.
56  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
57  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
58		  (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
59		  (x8/x16/x32/x64), "string", "ustring" and bitfield
60		  are supported.
61
62  (\*1) only for the probe on function entry (offs == 0).
63  (\*2) only for return probe.
64  (\*3) this is useful for fetching a field of data structures.
65  (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
66
67Types
68-----
69Several types are supported for fetch-args. Kprobe tracer will access memory
70by given type. Prefix 's' and 'u' means those types are signed and unsigned
71respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
72in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
73or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
74x86-64 uses x64).
75These value types can be an array. To record array data, you can add '[N]'
76(where N is a fixed number, less than 64) to the base type.
77E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements.
78Note that the array can be applied to memory type fetchargs, you can not
79apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
80wrong, but '+8($stack):x8[8]' is OK.)
81String type is a special type, which fetches a "null-terminated" string from
82kernel space. This means it will fail and store NULL if the string container
83has been paged out. "ustring" type is an alternative of string for user-space.
84See :ref:`user_mem_access` for more info..
85The string array type is a bit different from other types. For other base
86types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
87as +0(%di):x32.) But string[1] is not equal to string. The string type itself
88represents "char array", but string array type represents "char * array".
89So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
90Bitfield is another special type, which takes 3 parameters, bit-width, bit-
91offset, and container-size (usually 32). The syntax is::
92
93 b<bit-width>@<bit-offset>/<container-size>
94
95Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
96which shows given pointer in "symbol+offset" style.
97For $comm, the default type is "string"; any other type is invalid.
98
99.. _user_mem_access:
100
101User Memory Access
102------------------
103Kprobe events supports user-space memory access. For that purpose, you can use
104either user-space dereference syntax or 'ustring' type.
105
106The user-space dereference syntax allows you to access a field of a data
107structure in user-space. This is done by adding the "u" prefix to the
108dereference syntax. For example, +u4(%si) means it will read memory from the
109address in the register %si offset by 4, and the memory is expected to be in
110user-space. You can use this for strings too, e.g. +u0(%si):string will read
111a string from the address in the register %si that is expected to be in user-
112space. 'ustring' is a shortcut way of performing the same task. That is,
113+0(%si):ustring is equivalent to +u0(%si):string.
114
115Note that kprobe-event provides the user-memory access syntax but it doesn't
116use it transparently. This means if you use normal dereference or string type
117for user memory, it might fail, and may always fail on some archs. The user
118has to carefully check if the target data is in kernel or user space.
119
120Per-Probe Event Filtering
121-------------------------
122Per-probe event filtering feature allows you to set different filter on each
123probe and gives you what arguments will be shown in trace buffer. If an event
124name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
125under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
126'enable', 'format', 'filter' and 'trigger'.
127
128enable:
129  You can enable/disable the probe by writing 1 or 0 on it.
130
131format:
132  This shows the format of this probe event.
133
134filter:
135  You can write filtering rules of this event.
136
137id:
138  This shows the id of this probe event.
139
140trigger:
141  This allows to install trigger commands which are executed when the event is
142  hit (for details, see Documentation/trace/events.rst, section 6).
143
144Event Profiling
145---------------
146You can check the total number of probe hits and probe miss-hits via
147/sys/kernel/debug/tracing/kprobe_profile.
148The first column is event name, the second is the number of probe hits,
149the third is the number of probe miss-hits.
150
151Kernel Boot Parameter
152---------------------
153You can add and enable new kprobe events when booting up the kernel by
154"kprobe_event=" parameter. The parameter accepts a semicolon-delimited
155kprobe events, which format is similar to the kprobe_events.
156The difference is that the probe definition parameters are comma-delimited
157instead of space. For example, adding myprobe event on do_sys_open like below
158
159  p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)
160
161should be below for kernel boot parameter (just replace spaces with comma)
162
163  p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack)
164
165
166Usage examples
167--------------
168To add a probe as a new event, write a new definition to kprobe_events
169as below::
170
171  echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
172
173This sets a kprobe on the top of do_sys_open() function with recording
1741st to 4th arguments as "myprobe" event. Note, which register/stack entry is
175assigned to each function argument depends on arch-specific ABI. If you unsure
176the ABI, please try to use probe subcommand of perf-tools (you can find it
177under tools/perf/).
178As this example shows, users can choose more familiar names for each arguments.
179::
180
181  echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
182
183This sets a kretprobe on the return point of do_sys_open() function with
184recording return value as "myretprobe" event.
185You can see the format of these events via
186/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
187::
188
189  cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
190  name: myprobe
191  ID: 780
192  format:
193          field:unsigned short common_type;       offset:0;       size:2; signed:0;
194          field:unsigned char common_flags;       offset:2;       size:1; signed:0;
195          field:unsigned char common_preempt_count;       offset:3; size:1;signed:0;
196          field:int common_pid;   offset:4;       size:4; signed:1;
197
198          field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
199          field:int __probe_nargs;        offset:16;      size:4; signed:1;
200          field:unsigned long dfd;        offset:20;      size:4; signed:0;
201          field:unsigned long filename;   offset:24;      size:4; signed:0;
202          field:unsigned long flags;      offset:28;      size:4; signed:0;
203          field:unsigned long mode;       offset:32;      size:4; signed:0;
204
205
206  print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
207  REC->dfd, REC->filename, REC->flags, REC->mode
208
209You can see that the event has 4 arguments as in the expressions you specified.
210::
211
212  echo > /sys/kernel/debug/tracing/kprobe_events
213
214This clears all probe points.
215
216Or,
217::
218
219  echo -:myprobe >> kprobe_events
220
221This clears probe points selectively.
222
223Right after definition, each event is disabled by default. For tracing these
224events, you need to enable it.
225::
226
227  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
228  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
229
230Use the following command to start tracing in an interval.
231::
232
233    # echo 1 > tracing_on
234    Open something...
235    # echo 0 > tracing_on
236
237And you can see the traced information via /sys/kernel/debug/tracing/trace.
238::
239
240  cat /sys/kernel/debug/tracing/trace
241  # tracer: nop
242  #
243  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
244  #              | |       |          |         |
245             <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
246             <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
247             <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
248             <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
249             <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
250             <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
251
252
253Each line shows when the kernel hits an event, and <- SYMBOL means kernel
254returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
255returns from do_sys_open to sys_open+0x1b).
256